For me, first I crop the image to the size of the button or whatever it is. Next, I "Save For Web" or Shift+Ctrl+Alt+S for p.c. users with Photoshop 7.0. (I'm not sure if the hot keys are different for newer versions.
I use "Save For Web" because you can adjust little things like the quality, transparency (for gifs of course) et cetera, while comparing with the original image. And the pop-up screen (for "Save For Web") shows you the time it will take for the image to load as well. So that's helpful. Try fooling around with the options. It's the best way to see what you want and what you don't want.